Many modern automotive ignition systems include a control circuit operable to control a power transistor connected to an ignition coil. In such systems, the control circuit is operable to drive the power transistor so as to switch ignition coil current on and off, and to linearly control the magnitude of the coil current after the coil has been charged to the desired current level. Automotive ignition systems of this type typically use a current sensing resistor separate from the control circuit to detect the magnitude of the coil current, and to feed this signal back to the control circuit, wherein the control circuit is responsive to the sense resistor signal to dynamically limit the ignition coil current to a desired "hold" current level.
One example of a known automotive ignition system 10 of the type just described is shown in FIG. 1. System 10 includes a control computer 12, which is typically operable to control and manage the overall operation of an internal combustion engine as well as other vehicle subsystems, that is electrically connected to a control circuit 14. Control computer 12 is responsive to various engine/vehicle operating parameters, as is known in the art, to produce an electronic spark timing (EST) signal and provide this signal to control circuit 14. Control circuit 14 includes an EST input buffer 18 electrically connected to a gate drive control circuit 20, wherein gate drive control circuit 20 is operable to provide a gate drive voltage V.sub.GD to a power transistor 16. The power transistor 16 is connected at one end to an automotive ignition coil L1, the opposite end of which is connected to a suitable voltage source B+, and is also connected to a sense resistor R.sub.SENSE. The common connection between power transistor 16 and R.sub.SENSE is connected to an inverting input of an error amplifier 22 having a non-inverting input connected to a voltage reference VREF. The output of the error amplifier 22 is electrically connected to the gate drive control circuit 20.
In operation, the gate drive control circuit 20 is responsive to a rising edge of the EST signal, buffered by EST input buffer 18, to switch the gate drive voltage V.sub.GD from a low state to a suitable gate drive voltage as shown by voltage 32 of waveform 30 of FIG. 2. The power transistor 16 is responsive to the gate drive voltage 32 to begin conducting coil current I.sub.L through ignition coil L1. When the coil current I.sub.L reaches a pre-defined level, As shown by current level 38 of waveform 36, wherein the voltage across R.sub.SENSE is approximately equal to VREF, error amplifier 22 provides a corresponding signal to gate drive control circuit 20 to which circuit 20 is responsive to decrease gate drive voltage V.sub.GD to level 34 of waveform 30 to thereby limit I.sub.L to a predefined "hold" current level 38 or coil current limit level.
In practice, the resistor R.sub.SENSE is adjusted, or "trimmed", to cancel variations in the threshold voltage of power transistor 16 and other variations in control circuit 14 to thereby set the coil current limit to a specified level. If this trimming operation is not performed, manufacturing process related variations in power transistor 16, control circuit 14, and current sense resistor R.sub.SENSE can result in an excessively wide range of coil current limit levels within a given sample of power transistors 16 and control circuits 14.
In some applications of system 10, certain package and/or circuit configurations do not allow for trimming of the current sense resistor R.sub.SENSE after the control circuit 14 is mated with a randomly chosen power transistor 16. Variations internal to control circuit 14, however, can be adjusted, or "trimmed out", during testing and/or calibration of the control circuit 14. During such a trimming process, control circuit 14 is typically configured to produce a specific gate drive output voltage V.sub.GD which is selected to be approximately the average expected gate drive voltage required to achieve a desired coil current limit level for a particular type of power transistor 16. However, since randomly chosen power transistors 16 exhibit a distribution of voltages required to set conduction at a specific current level, the combination of "trimmed out" control circuits 14 with randomly chosen power transistors 16 will subject the completed systems 10 to variations in the coil current limit levels.
Typically, power transistor 16 is provided as an insulated gate bipolar transistor (IGBT) which has a gate voltage-to-collector current relationship, or "transconductance", that varies with temperature. Use of an IGBT for power transistor 16 thus introduces additional variations in the gate voltage required to "hold" the coil current I.sub.L at the desired coil current limit level.
Control circuit 14 typically includes feedback control loops having selectable gain that is capable of removing most of the effects of the foregoing variations in gate voltage V.sub.GD. However, these variations further cause feedback currents in the feedback control loops to vary, thereby resulting in internal bias changes within control circuit 14 that cause I.sub.L to deviate from the desired coil current limit level.
Deviations in the coil current limit levels, as just described, are unacceptable and should be eliminated. As with any closed loop dynamic control system, however, too much gain in the feedback control loops of circuit 14 may result in circuit instability. It is therefore desirable to remove the bias effects internal to circuit 14 that are due to varying gate voltage levels V.sub.GD by a technique other than increasing the gain in the error amplifier gain loop.